Radio frequency (RF) communication systems, such as aeronautical satellite communications, produce active intermodulation (IM) or passive intermodulation ((P)IM) emissions whenever two or more signals at different frequencies are transmitted simultaneously. The (P)IM emissions, which are caused when physical parts begin to degrade, can generate harmful interference to other radio receivers on the same or nearby aircraft, such as receivers in satellite navigation systems. To manage or prevent such interference (for which there often are regulatory requirements), many techniques have been employed, and a built-in test (BIT) may be required to verify compliance with regulatory requirements.
One test technique for monitoring such emissions is to transmit two (or more) test signals simultaneously at different pre-determined frequencies and then measure RF signal energy within a certain bandwidth around one or more pre-determined receive frequencies that correlate to certain IM product orders (3rd, 5th, 7th, etc.) that result from the transmit frequencies. Prior approaches have included manual or automated testing involving the transmission of multiple simultaneous time-continuous test signals (typically two, and typically continuous wave (CW)), then measuring with a spectrum analyzer (or an embedded digital signal processor equivalent) for (P)IM signals above the noise floor at the pre-determined frequencies of the various (P)IM products. The measurements are compared to regulatory requirements, nominal performance levels, or other pre-determined performance levels in order to determine whether or not a (P)IM problem exists. The simultaneity of the test transmissions is guaranteed.
Rather than transmit specific test signals, multiple normal operation transmissions can be used at different frequencies to continuously monitor the (P)IM emissions of a system. However, as (P)IM only occurs when the multiple transmissions occur simultaneously, the (P)IM test measurements must normally be done when the transmissions are known to be occurring simultaneously, and then compared to measurements taken when transmissions are known to not be occurring simultaneously, in order to determine that interference emissions are occurring and that they are in fact due to (P)IM.
Moreover, there are problems with trying to use normal operation transmissions for any such testing, including on/off bursting of the transmit frequencies at non-predictable random or pseudo-random time intervals, which can be of very short on-time duration, thus making the determination of exactly when simultaneous transmissions are occurring very difficult, if not impossible, to predict in advance. The inherent burstiness of some such transmit channels can make it impossible to have simultaneous transmission occur for a long-enough period to arrange for test measurements to be made during the time of simultaneity. This situation can be exacerbated by the specific system architecture, which may involve multiple hardware units interconnected by relatively low speed communications interfaces for built-in test and/or normal operations data, which further reduces the possibility of being able to coordinate test measurement sampling to occur at the required times of simultaneity.
As a result of these obstacles, system designers may be forced to follow the multiple test signal approach, with its probable higher implementation costs, requirement for dedicated time periods for test execution, reservation of RF spectrum for the test transmissions, possible reservation of RF spectrum for “quiet” receive test frequencies, restrictions on the effective isotropic radiated power (EIRP) levels of the test transmissions (which may depend on the direction that the test signal power is radiated relative to satellites or other signal receivers), and the like.